This invention generally relates to display systems that form a two-dimensional image and more particularly relates to a color display apparatus and method for generating images having a broadened color gamut using electromechanical grating devices.
With the advent of digital technology and the demonstration of all-digital projection systems, there is considerable interest in increasing the range or gamut of colors that can be displayed in order to provide a more realistic, more vivid image than is possible with the gamut limitations of film dyes or phosphors. The familiar tristimulus CIE color model developed by Commission International de l""Eclairage (International Commission on Illumination) shows the color space perceived by a standard human observer. FIG. 1a shows the CIE color model, which represents a visible gamut 200 as a familiar xe2x80x9chorseshoexe2x80x9d curve. Within visible gamut 200, the gamut of a conventional display device can be represented by a three-sided device gamut 202, such as standard SMPTE (Society of Motion Picture and Television Engineers) phosphors, for example. As is well known in the color projection arts, it is desirable for a display device to provide as much of visible gamut 200 as possible in order to faithfully represent the actual color of an image.
Referring to FIG. 1a, pure, saturated spectral colors are mapped to the xe2x80x9chorseshoexe2x80x9d shaped periphery of visible gamut 200. The component colors of a display, typically Red, Green, and Blue (RGB) define the vertices of the polygon for a color gamut, thereby defining the shape and limits of device gamut 202. Ideally, these component colors are as close to the periphery of visible gamut 200 as possible. The interior of the xe2x80x9chorseshoexe2x80x9d then contains all mappings of mixtures of colors, including mixtures of pure colors with white, such as spectral red with added white, which becomes pink, for example.
One simple strategy to increase the size of device gamut 202 is to use light sources that are spectrally pure, or have at least a good degree of spectral purity. Lasers, due to their inherent spectral purity, are particularly advantaged for maximizing device gamut 202. A second strategy for expanding color gamut is to move from the conventional triangular area of device gamut 202, as shown in FIG. 1a, to a polygonal area, shown as an expanded device gamut 204 in FIG. 1b. In order to do this, one or more additional component colors must be added.
There have been projection apparatus solutions proposed that may employ more than 3 component colors from various color light sources. For the most part, however, the solutions proposed have not targeted color gamut expansion; in some cases, added colors are not selected for spectral purity, but are selected for some other characteristic. Disclosures of projectors using more than three component color sources include the following: U.S. Pat. No. 6,256,073, Jul. 3, 2001 (Pettitt) discloses a projection apparatus using a filter wheel arrangement that provides four colors in order to maintain brightness and white point purity. However, the fourth color added in this configuration is not spectrally pure, but is white in order to add brightness to the display and to minimize any objectionable color tint. It must be noted that white is an xe2x80x9cintragamutxe2x80x9d color addition; in terms of color theory, adding white can actually reduce the color gamut. Similarly, U.S. Pat. No. 6,220,710 by Raj et al. issued Apr. 24, 2001 discloses the addition of a white light channel to standard R, G, B light channels in a projection apparatus. As was just noted, the addition of white light may provide added luminosity, but constricts the color gamut. U.S. Pat. No. 6,191,826 Feb. 20, 2001 (Murakami et al.) discloses a projector apparatus that uses four colors derived from a single white light source, where the addition of a fourth color, orange, compensates for unwanted effects of spectral distribution that affect the primary green color path. In the apparatus of U.S. Pat. No. 6,191,826, the specific white light source used happens to contain a distinctive orange spectral component. To compensate for this, filtering is used to attenuate undesirable orange spectral content from the green light component in order to obtain a green light having improved spectral purity. Then, with the motive of compensating for the resulting loss of brightness, a separate orange light is added as a fourth color. The disclosure indicates that some expansion of color range is experienced as a side effect. However, with respect to color gamut, it is significant to observe that the solution disclosed in U.S. Pat. No. 6,191,826 does not appreciably expand the color gamut of a projection apparatus. In terms of the color gamut polygon described above with reference to FIGS. 1a and 1b, addition of an orange light may add a fourth vertex; however, any added orange vertex would be very close to the line already formed between red and green vertices. Thus, the newly formed gamut polygon will, at best, exhibit only a very slight increase in area over the triangle formed using three component colors. Moreover, unless a substantially pure wavelength orange is provided, there could even be a small decrease in color gamut using the methods disclosed in U.S. Pat. No. 6,191,826.
It is worthwhile to note that none of the solutions listed above has targeted the expansion of the color gamut as a goal or disclosed methods for obtaining an expanded color gamut. In fact, for each of the solutions listed above, there can even be some loss of color gamut with the addition of a fourth color.
In contrast to the above patent disclosures, Patent Application WO 01/95544 A2 (Ben-David et al.) discloses a display device and method for color gamut expansion as shown in FIG. 1b using spatial light modulators with four or more substantially saturated colors. In one embodiment, Application WO 01/95544 teaches the use of a color wheel for providing each of the four or more component colors to a single spatial light modulator. In an alternate embodiment, this Application teaches splitting light from a single light source into four or more component colors and the deployment of a dedicated spatial light modulator for each component color. However, while the teaching of Application WO 01/95544 may show devices that provide improved color gamut, there are several drawbacks to the conventional design solutions disclosed therein. When multiplexing a single spatial light modulator to handle more than three colors, a significant concern relates to the timing of display data. The spatial light modulator employed must provide very high-speed refresh performance, with high-speed support components in the data processing path. Parallel processing of image data would very likely be required in order to load pixel data to the spatial light modulator at the rates required for maintaining flicker-free motion picture display. It must also be noted that the settling time for conventional LCD modulators, typically in the range of 10-20 msec for each color, further shortens the available projection time and thus constrains brightness. Loss of brightness, already an acknowledged disadvantage of tricolor color wheel solutions, becomes a successively larger problem with each additional color added, since a smaller proportionate amount of light is then available for any one color. Moreover, the use of a filter wheel for providing the successive component colors at a sufficiently high rate of speed has further disadvantages. Such a filter wheel must be rotated at very high speeds, requiring a precision control feedback loop in order to maintain precision synchronization with data loading and device modulation timing. The additional xe2x80x9cdead timexe2x80x9d during filter color transitions, already substantial in devices using three-color filter wheels, would further reduce brightness and complicate timing synchronization. Coupling the filter wheel with a neutral density filter, also rotating in the light path, introduces additional cost and complexity. Although rotating filter wheels have been adapted for color projection apparatus, the inherent disadvantages of such a mechanical solution are widely acknowledged. Alternative solutions using a spatial light modulator dedicated to each color introduce other concerns, including proper alignment for component colors. The disclosure of Application WO 01/95544 teaches the deployment of a separate projection system for each color, which would be costly and would require separate alignment procedures for each display screen size and distance. Providing illumination from a single light source results in reduced brightness and contrast. Thus, while the disclosure of Application WO 01/95544 teaches gamut expansion in theory, in practice there are a number of significant drawbacks to the design solutions proposed. As a studied consideration of Application WO 01/95544 clearly shows, problems that were difficult to solve for three-color projection, such as timing synchronization, color alignment, maintaining brightness and contrast, cost of spatial light modulators and overall complexity, are even more challenging when attempting to use four or more component vertex colors.
Currently, promising solutions for digital cinema projection and home theater systems employ, as image forming devices, one of two types of area spatial light modulators (SLMs). An area spatial light modulator has a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate transmitted or reflected light from a light source. There are two salient types of area spatial light modulators that are conventionally employed for forming images in digital projection and printing apparatus: Digital Micro-Mirror Devices (DMDs) and Liquid-Crystal Devices (LCDs).
Prototype projectors using one or more DMDs have been demonstrated. DMD devices are described in a number of patents, for example U.S. Pat. No. 4,441,791 issued Apr. 10, 1984 to Hornbeck; U.S. Pat. No. 5,535,047 issued Jul. 9, 1996 to Hornbeck; U.S. Pat. No. 5,600,383 issued Feb. 4, 1997 to Hornbeck; and U.S. Pat. No. 5,719,695 issued Feb. 17, 1998 to Heimbuch. Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 issued Jun. 22, 1999 to Tejada et al.; U.S. Pat. No. 5,930,050 issued Jul. 27, 1999 to Dewald; U.S. Pat. No. 6,008,951 issued Dec. 28, 1999 to Anderson; and U.S. Pat. No. 6,089,717 issued Jul. 18, 2000 to Iwai. LCD apparatus are described, in part, in U.S. Pat. No. 5,570,213 issued Oct. 29, 1996 to Ruiz et al. and U.S. Pat. No. 5,620,755 issued Apr. 15, 1997 to Smith, Jr. et al.
While there has been some success with respect to RGB color representation using area spatial light modulators, there are significant limitations. Device timing and refresh cycles, for example, can be pushed to the limit for display apparatus using three source colors. Adapting a single LCD device to handle additional colors requires faster image data handling, allows even less refresh time, and reduces overall brightness. Using multiple LCDs can be costly and introduces alignment problems. LCDs exhibit a number of uniformity problems due to interference when used with laser sources. LCDs also present a particular challenge with respect to image contrast; it can be difficult to achieve the needed brightness levels, yet prevent stray light from the image path. Stray light prevents such devices from achieving the high quality black available with conventional film-based display devices. In summary, conventional approaches using LCDs and other two-dimensional spatial light modulators do not appear to offer the best solutions for providing display apparatus with a color gamut expanded from the existing 3-color model.
Linear arrays, which could also be considered as one-dimensional spatial light modulators, have some advantages over the two-dimensional LCD and DMD area spatial light modulators described above. Inherent performance advantages for linear arrays include the capability for higher resolution, reduced cost, and simplified illumination optics. Of particular interest: linear arrays are more suitable modulators for laser light than are their two-dimensional counterparts. For example, Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 issued May 10, 1994 to Bloom et al. are an earlier type of linear array that offers a workable solution for high-brightness imaging using laser sources.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed in U.S. Pat. No. 6,307,663 issued Oct. 23, 2001 to Kowarz, entitled xe2x80x9cSpatial Light Modulator With Conformal Grating Devicexe2x80x9d. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of U.S. Pat. No. 6,307,663 has more recently become known as the conformal GEMS device, with GEMS standing for Grating ElectroMechanical System. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system. When used with laser sources, GEMS devices provide excellent brightness, speed, and contrast.
U.S. Pat. No. 6,411,425 issued Jun. 25, 2002 to Kowarz et al. discloses an imaging system employing GEMS devices in a number of printing and display embodiments. As with its GLV counterpart, a GEMS device modulates a single color and a single line of an image at a time. Thus, forming a color image requires suitable techniques either for sequencing illumination and modulation data for each color to a single linear modulator or for combining separately modulated color images. With conventional RGB color systems, various techniques have been developed and used for color-sequential image-forming using three colors as well as for combining three separately modulated images. However, it can be appreciated that there are significant challenges in adapting these devices to a color display apparatus for providing images using more than three colors to produce an expanded color gamut.
In spite of the shortcomings of prior art solutions, it is recognized that there would be significant advantages in providing an image display having an expanded color gamut. Natural colors could be more realistically reproduced. At the same time, computer-generated images, not confined to colors and tones found in nature, could be represented more dramatically. Thus, there is a need for an improved imaging solution for viewing images having an expanded color gamut, where the solution provides a structurally simple apparatus, minimizes aberrations and image distortion, and meets demanding requirements for high contrast, high brightness, high speed, flexible operation, and lowered cost.
The present invention is directed to overcoming one or more of the problems set forth above by providing an improved imaging apparatus having an expanded color gamut defined by more than three vertex colors for forming a color image on a display surface, that includes: (a) a light source comprising at least one dichroic combiner for transmitting a first vertex color and reflecting a second vertex color toward a color combining element that directs, along an illumination axis, a colored illumination beam having, at one time, any one of at least four different vertex colors; (b) a linear array of electromechanical grating devices for receiving the colored illumination beam along the illumination axis; (c) an obstructing element for blocking a zeroth order light beam reflected from the linear array of electromechanical grating devices from reaching the display surface; (d) a projection lens cooperating with a scanning element for directing at least one diffracted light beam from the linear array of electromechanical grating devices toward the display surface, thereby forming a line image of the linear array on the display surface; and (e) a logic control processor for controlling timing of the light source and for providing image data to the linear array of electromechanical grating devices.
The present invention also provides a method for providing an expanded color gamut defined by more than three substantially monochromatic vertex colors for an imaging apparatus that forms a color image on a display surface, comprising: (a) providing, along an illumination axis, a colored illumination beam having, at any one time, any of at least four different vertex colors; (b) positioning a linear array of electromechanical grating devices for receiving the colored illumination beam along the illumination axis; (c) blocking a zeroth order of a light beam reflected from the linear array of electromechanical grating devices from reaching the display surface; (d) projecting a plurality of diffracted orders of the light beam toward a scanning element for directing the plurality of diffracted orders of the light beam toward the display surface; and, (e) controlling timing of the colored illumination beam and image data provided to the linear array of electromechanical grating devices.
It is an advantage of the present invention that it utilizes the inherent high resolution, high efficiency, and high contrast capabilities of both GLV and conformal GEMS devices.
It is a further advantage of the present invention that it employs image-forming devices capable of achieving the high brightness levels needed for large-scale display and projection applications.
It is a further advantage of the present invention that it allows expansion of color gamut by selective use of four or more source colors. The present invention also allows a display apparatus, with appropriate timing changes, the option to switch between number of colors used, depending on scene content, for example. The present invention also allows the option to alter the domain of a color gamut by selecting a subset of the available vertex colors.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.